1. Background and Technical Problems
Mechanical drive trains in which a transmission and clutch assembly cooperate to transmit engine torque to a vehicle drive shaft are generally well known. For example, transmission systems in motorcycles typically include an engine having an output crankshaft which cooperates with a clutch mechanism for selectively applying torque to a transmission input shaft in accordance with the position of the clutch. When the clutch is engaged, the crankshaft drives a clutch hub which, in turn, drives the transmission input shaft. The transmission converts input torque from the transmission input shaft into output torque at a transmission output shaft in accordance with one of a plurality of gears housed within the transmission. The amount of torque ultimately transmitted to the vehicle wheel sprocket depends, inter alia, on the particular gear selected.
When the clutch is disengaged, the power transmission circuit is temporarily interrupted so that no torque is applied to the transmission input shaft. This allows the transmission to change from one gear to another, whereupon the clutch may be re-engaged to again complete the power transmission circuit.
The transmission input shaft is circumscribed by the clutch hub intermediate the engine and transmission. Various configurations have been employed to insure that the hub and shaft maintain driving engagement with respect to each other while minimizing power losses during torque transmission. For example, certain systems employ a keyway in the inner diameter of the hub, there being an interlocking "key" extending from the outer diameter of the shaft into the keyway. Alternatively, a tapered shaft, having a conical or frustoconical portion frictionally engaging a similarly shaped hub portion has been employed, whereby the shaft is loaded axially into the hub to maintain driving engagement between the respective mating tapered surfaces.
The most common shaft/hub interface comprises a plurality of equally spaced splines disposed about the outer diameter of the shaft near an end thereof, and corresponding mating splines disposed about the inner diameter of the hub for driving engagement with the shaft splines. The splines are advantageously disposed along a series of straight lines parallel to the common axis of the shaft and hub.
Design tolerances associated with the splines facilitate assembly of the system and enhance the interchangeability of the respective parts. Consequently, a small amount of clearance is built into the spline interface, such that a limited amount of angular motion is permitted between the shaft and hub despite the splined engagement therebetween.
For example, if the hub is held stationary and the shaft is positioned therewithin such that the nominal peak center of each shaft spline is aligned with the nominal valley center of each spline, the shaft may be rotated by a small amount .theta..sub.1 in a first direction or, alternatively, by a small amount .theta..sub.2 in the opposite direction, until the splines contact each other. Thus, a total nominal angular clearance of .theta..sub..tau. =.theta..sub.1 +.theta..sub.2 is designed into each shaft/hub assembly. Of course, the actual amount of angular clearance associated with a particular assembly may be somewhat greater or less than .theta..sub..tau..
In order to transmit torque to the vehicle wheel sprocket, the clutch must be engaged to permit engine torque to be applied to the transmission via the transmission input shaft. In addition, one of the gears must be selected to permit torque to be transmitted from the transmission input shaft to the transmission output shaft. The transmission output shaft transmits torque to the vehicle wheel, either directly or, for example, through a chain or belt to a sprocket mounted on the wheel. When the clutch is engaged, the angular clearance is taken up as the hub splines are loaded against the shaft splines, and the hub and shaft rotate together.
When the transmission is in neutral, on the other hand, no torque is transmitted from the transmission input shaft to the transmission output shaft. Thus, the end of the transmission input shaft remote from the clutch hub is essentially unconstrained. When the clutch is also disengaged, both ends of the transmission input shaft are unconstrained. In this condition, the shaft splines typically are not angularly loaded against the hub splines except, perhaps, for loading due to gravity or due to "winding down" upon being shifted from one of the gears into neutral. In any case, revolutions (rpms) of the crankshaft are not transmitted to the hub/shaft splined interface.
In contrast, engine rpms are transmitted to the hub/shaft interface when the clutch is engaged. As stated above, engine torque is transmitted to the vehicle wheel when the vehicle is in gear. However, when the clutch is engaged but the transmission is in neutral (the vehicle is idling), engine rpms are transmitted to the transmission input shaft yet no torque is transmitted thereby because the end of the shaft opposite the clutch is unconstrained. As a result, the design clearance .theta..sub..tau. associated with the hub/shaft splined interface permits a limited degree of relative angular movement between the shaft and the hub, giving rise to the possibility of "rattle" observed in some motorcycles at idle. Over time this relative angular motion can result in an increase in the angular clearance .theta..sub..tau. through the phenomenon of fretting corrosion, i.e., corrosion produced by the relative motion at the hub/shaft interface due to either too little control or too much control.
Early attempts to control this relative angular movement involved the use of a circular clip disposed about the end of the shaft extending through the hub. The clip was seated within a circular groove disposed about the shaft and had a flat bearing surface configured to contact an oppositely disposed bearing surface on the hub. Alternatively, a washer was disposed about the shaft between the clip and the hub. Properly dimensioned, this system was capable of transmitting a retaining load to the shaft, which load was transmitted to the bearing surface at the clip/hub (or washer/hub) interface, thereby reducing the degree of relative motion between the shaft and the hub. However, due to the cumulative effect of dimensional variations (tolerance "stack-up") in the various components, it has proven difficult to control the amount of friction at the clip/hub (or washer/hub) interface.
Further attempts have involved the introduction of a nut threadedly engaging the distal end of the shaft. The nut and hub are provided with cooperating bearing surfaces to limit the relative angular motion between the shaft and hub in much the same way as the above-mentioned clip, with the advantage that the amount of bearing force could be adjusted as a function of the seating torque applied to the nut. However, it has been observed that the ability of the nut to control relative angular motion decreases over time. Moreover, due to the stiffness of the mechanical circuit, it has proven difficult to control the amount of friction at the nut/hub interface.
Accordingly, a mechanism is needed for controllably applying a predetermined frictional force to the shaft to thereby control the extent of relative angular motion between the shaft and hub to within desired limits, and which mechanism remains effective over time.
2. Summary of the Problem
The present inventor has determined that presently known mechanisms for controlling the relative angular motion between the shaft and hub which employ a nut secured to the distal end of the shaft are susceptible to cyclic acceleration reversals which tend to decrease the frictional "control" force over time. Due to the inherent stiffness of the retaining system, small changes in seating torque have a pronounced effect on the amount of friction available for controlling relative angular motion between the shaft and the hub. In addition, consistent control of the frictional forces between the hub and shaft is particularly problematic because of the cumulative effects of piece-part dimensional variations.
3. Summary of the Solution
In accordance with one aspect of the present invention, methods and apparatus are provided for damping the effects of cyclic engine speed variations, thereby enhancing control of the relative angular motion between the splines of a driving hub and the mating splines of a shaft driven by the hub while the vehicle is at idle.
A particularly advantageous feature of the present invention provides a caliper-like retaining mechanism for applying a predetermined resistance to relative motion between the shaft and hub, which resistance is selected to be sufficient to adequately inhibit fretting corrosion. In addition, the caliper mechanism interrupts the stiff mechanical circuit comprising the hub shoulder and retaining nut shoulder, allowing the control system of the present invention to remain effective over time.
A specific implementation of the present invention provides a retaining nut threadedly engaged to the distal end of a driven shaft, which distal end is journalled through a splined bore of a driving hub. In prior art systems, the nut is configured to abut an annular shoulder disposed on the face of the hub in a plane transverse (i.e., not parallel) to the common axis of the shaft and hub. The present system, in contrast, provides a shoulder washer disposed about the shaft proximate the hub shoulder, which shoulder washer is secured to the shaft by the retaining nut. A disc spring is provided for urging the shoulder washer into frictional engagement with the hub, the disc spring being retained by a snap ring advantageously seated in a circular groove within the hub. An important feature of the disc spring is that it exhibits an essentially constant spring force over a predetermined range of compression. Consequently, small changes in disc spring compression do not appreciably affect disc spring preload, resulting in enhanced frictional control.
The relative sizes and disposition of the respective elements, in conjunction with the configuration of the disc spring, yields a predetermined preload exerted by the disc spring on the shoulder washer, which preload is sufficient to overcome the forces which tend to induce rattle between the splines of the shaft and the corresponding splines of the hub. At the same time, the washer, spring, and clip interrupt the mechanical circuit which could otherwise influence the initial seating torque on the nut as the nut/hub interface is subjected to acceleration reversals during vehicle operation.